1. Field of the Invention
The present invention relates to wireless integrated circuit (IC) devices including a wireless IC and a radiation plate. More particularly, the present invention relates to a wireless IC device preferably for use in a radio frequency identification (RFID) system performing communication using an HF band frequency.
2. Description of the Related Art
In recent years, an RFID system has been developed as an article management system in which electromagnetic-field-based contactless communication is performed between a reader/writer generating an induction field and a wireless IC tag (hereinafter, also referred to as a wireless IC device) affixed to an article and storing predetermined information so that the predetermined information is transmitted.
The wireless IC tag used in this RFID system includes a wireless IC chip that processes a predetermined radio signal and a radiation plate that transmits and receives the radio signal. For example, a wireless IC tag disclosed in Japanese Unexamined Patent Application Publication No. 2007-102348 is known.
The wireless IC tag disclosed in Japanese Unexamined Patent Application Publication No. 2007-102348 is constituted by a multilayer antenna pattern and an IC chip. Swirling electrodes are disposed on a plurality of layers to form the antenna pattern. A resonance circuit is constituted by inductance generated by the electrodes and inter-electrode capacitance and capacitance of the IC chip. The resonant frequency of this resonance circuit is set equal to communication frequency, e.g., 13.56 MHz. The wireless IC tag communicates with a reader/writer through the antenna pattern.
However, the wireless IC tag has the following problems. Since the antenna pattern is covered with a protection film but is exposed to the outside, a magnetic field generated by the antenna pattern leaks to the outside and an inductance value of the antenna pattern changes because of influences of the dielectric constant and the shape of articles attached with the tag. Variance of the resonant frequency due to the inductance value change causes a communication failure.
To prevent the magnetic field from leaking to the outside and to increase the inductance value, the antenna pattern may be disposed in a magnetic body, such as ferrite. However, when the antenna pattern is disposed completely within the magnetic body, the magnetic field is trapped inside the magnetic body and communication cannot be performed.